1. Field of the Invention
The present invention relates to a lens barrel and an optical apparatus.
2. Description of the Related Art
In a single lens reflex camera system, there is known a lens system that extends a focal length by inserting an intermediate adaptor having an extender optical system configured to shift the focal length to a telephoto side between a camera body and an interchangeable lens. In a TV camera system, there is also known an optical system that includes an extender exchanger of changing a focal length of the entire system by inserting or retreating a lens unit having an extender optical system into or from a part of an optical path of an imaging lens (zoom lens).
In order to attach the intermediate adaptor to the camera body, it is necessary to detach the interchangeable lens from the camera body and to install the intermediate adaptor to the interchangeable lens. However, this manipulation is unnecessary for an extender built-in exchanger type. Therefore, in order to employ an extender built-in exchanger type in a single lens reflex camera system, Japanese Patent Laid-Open No. (“JP”) 06-201979 proposes a configuration of inserting or detaching an extender optical system into or from an optical path between a front relay-lens unit and a rear relay-lens unit constituting a zoom lens.
However, in the configuration of JP 06-201979, in order to detach the extender optical system, one of the front and rear relay-lens units needs to be detached. However, detaching of one of the front and rear relay-lens units would disadvantageously change the optical performance of an image-pickup optical system.